What Happens When the Zygote Divides? On the Metaphysics of Monozygotic Twinning.

It is often argued that certain metaphysical complications surrounding the phenomenon of monozygotic twinning force us to conclude that, prior to the point at which twinning is no longer possible, the zygote or early embryo cannot be considered an individual human organism. In this essay, I argue, on the contrary, that there are in fact several ways of making sense of monozygotic twinning that uphold the humanity of the original zygote, but also that there is no easy answer to what happens when the human zygote twins. All of the options available carry with them one or more surprising, alarming, or otherwise counterintuitive implications. All things considered, I conclude that the "budding option," according to which the original human organism present before twinning carries on as one of the resulting embryos but not the other, is the most plausible explanation of what happens when a human zygote twins.

Long-Term Neurodevelopmental Outcome After Selective Fetoscopic Laser Therapy for Stage I Twin-Twin Transfusion Syndrome.

Twin-twin transfusion syndrome (TTTS) is a serious complication in monochorionic (MC) pregnancies. Fetoscopic laser surgery (FLS) is the primary treatment for advanced TTTS, but managing Quintero stage I TTTS is still controversial. We conducted an observational study evaluating the 2-year neurodevelopment of fetuses, which underwent FLS for stage I TTTS, compared with advanced TTTS and uncomplicated monochorionic diamniotic twins (MCDTs). The study included 156 children: 14 in stage I TTTS group, 28 in advanced TTTS group, and 114 in uncomplicated twin group. In stage I TTTS, 92.9% showed normal neurodevelopment, with no severe neurological impairments observed. These results were comparable with uncomplicated twins (92.1% normal neurodevelopment, P = .921, adjusted odds ratio [aOR] = 1.56, 95% confidence interval [CI] = 0.42-5.79; 1.8% severe impairment, P = .617). Advanced TTTS had a non-significant lower rate of normal neurodevelopment (89.3%, P = .710, aOR = 1.31, 95% CI = 0.12-14.87). In conclusion, FLS for stage I TTTS shows favorable long-term neurodevelopmental outcomes, similar to uncomplicated MC pregnancies.

Effect of blastocyst morphology on the incidence of monozygotic twinning pregnancy after single blastocyst transfer.

Aims: To explore whether blastocyst morphology [blastocyst stage, inner cell mass (ICM), and trophectoderm (TE) grading] impacts the occurrence of monozygotic twinning (MZT) after single blastocyst transfer (SBT).Materials and methods: A single-center retrospective cohort study was conducted including all clinical pregnancies after single blastocyst transfer between January 2015 and September 2021 (n = 9229). Blastocyst morphology was assessed using Gardner grading system. MZT was defined as more than one gestational sac (GS), or two or more fetal heartbeats in a single GS via ultrasound at 5-6 gestational weeks.Results: The overall MZT rate was 2.46% (227 of 9229 cases), of which was the highest in blastocysts of grade A TE and lowest in those with grade C TE (grade A: B:C = 3.40%:2.67%:1.58%, p = .002). Higher risk of MZT pregnancy was associated with higher trophectoderm grading [A vs. C: aOR, 1.883, 95% CI 1.069-3.315, p = .028; B vs C: aOR, 1.559, 95% CI 1.066-2.279, p = .022], but not extended culture in vitro (day 5 vs. day 6), vitrification (fresh vs. frozen-thawed ET), assisted hatching (AH), blastocyst stage (stage 1-6) or ICM grading (A vs. B).Conclusions: We conclude that TE grade is an independent risk factor of MZT after single blastocyst transfer. Blastocysts with high-grade trophectoderm are more liable to obtain monozygotic multiple gestation.

Assisted Reproduction and Discussion of Rare Cases in Monozygotic Twinning.

Assisted reproductive technology is a crucial factor that increases the incidence of monozygotic twinning in humans. This article discusses the impact of various indicators in assisted reproductive technology studies on pregnancy outcomes, especially studies with a large number of clinical cases. Furthermore, three rare cases in multiples pregnancy are discussed: fetus papyraceous of a pair of male monozygotic twins in a set of triplets, two pairs of sesquizygotic twins with sex-discordance, and rare conjoined triplets.

Odds and Predictors of Monozygotic Twinning in a Multicentre Cohort of 25,794 IVF Cycles.

The rate of monozygotic twinning (MZT) has seen a gradual increase in recent years. Numerous parameters involved in ART procedures are blamed for this surge, even though the exact explanation is as yet unknown. Our study's objectives were to determine the risk variables for monozygotic twinning after ART and to estimate their prevalence. We examined 25,794 IVF cycles for the incidence of monozygotic twinning in this observational analysis. Our study, which was carried out across seven tertiary IVF centres over the course of four years, found an overall MZT rate of 0.37% per embryo transfer procedure and 0.88% of all pregnancies. Monozygotic twinning was more commonly seen in fresh single-embryo transfer (SET) and blastocyst transfer cycles. Larger multicentre studies are needed to explore the potential risk variables.

The Impact of Assisted Hatching on Monozygotic Twinning is Related to Female Age and Insemination Method: A New Perspective.

Whether assisted hatching (AH) is associated with a higher incidence of monozygotic twinning (MZT) in women undergoing assisted reproductive technology remains controversial; the aim of the study was to demonstrate the relationship between AH and MZT. A total of 8900 clinical pregnancies were selected among embryo transfer cycles from January 2011 to October 2019. Women receiving day (D) 3 embryos were divided into groups A-C: group A (n = 1651) and group B (n = 1045) included women aged ≤37 or ≥38 years, respectively, with zona pellucida (ZP) thinning; group C (n = 3865) included women aged ≤37 years without AH. Women aged ≤37 years who underwent blastocyst transfer and/or blastocyst ZP breaching were included in group D (n = 2339). The incidence of MZT was compared among groups A, B and C, and between groups C and D. The incidence of MZT in group B (2.2%) was significantly higher than in group A (1.0%), especially following intracytoplasmic sperm injection (ICSI), while the incidence of MZT in group A (1.0%) was significantly lower than in group C (2.2%). The MZT rate with in vitro fertilization was higher in group D (2.8%) than in group C (2.2%), but the MZT rate following ICSI was not significantly different between the two groups. ZP thinning of D3 embryos may increase the risk of MZT in older women (≥38 years), but decrease it in younger women (≤37 years). ZP breaching may be useful to reduce the incidence of MZT in ICSI-generated blastocysts.

Blastocyst morphology is associated with the incidence of monozygotic twinning in assisted reproductive technology.

BACKGROUND: An increased incidence of monozygotic twinning after a blastocyst transfer has been previously reported in assisted reproductive technology treatment. It is uncertain whether this phenomenon is due to the extended culture time, culture medium, or inherent blastocyst parameters. OBJECTIVE: This study aimed to investigate the association between blastocyst parameters (in vitro culture time, blastocyst stage, and inner cell mass and trophectoderm grading) and the incidence of monozygotic twinning after assisted reproductive technology. STUDY DESIGN: This was a retrospective cohort study employing data from a multicenter, large, electronic database from 4 academic hospitals. All clinical pregnancies after a single blastocyst transfer between January 2014 and February 2020 were included. Blastocyst morphology was evaluated based on the Gardner grading system, considering the blastocyst stage, and inner cell mass and trophectoderm grading (grades A, B, and C). Monozygotic twinning was defined as ≥2 fetal heartbeats in a single gestational sac or 2 gestational sacs with sex concordance at birth. The multivariable predicted marginal proportions from logistic regression models were used to compute adjusted relative risks for the association between blastocyst parameters and the incidence of monozygotic twinning. RESULTS: The overall monozygotic twinning rate was 1.53% (402 of 26,254 cases). The monozygotic twinning was not associated with the culture time in vitro (day 5 vs day 6) or blastocyst stage (early, blastocyst, expanded, hatching, and hatched). Alternatively, monozygotic twinning was associated with lower inner cell mass grading (B vs A: adjusted relative risk, 1.67 [95 % confidence interval, 1.28-2.25]; C vs A: adjusted relative risk, 1.98 [95% confidence interval, 1.18-3.11]) and higher trophectoderm grading (B vs C: adjusted relative risk, 1.38 [95% confidence interval, 1.03-1.92]; A vs C: adjusted relative risk, 2.14 [95% confidence interval, 1.45-3.20]). The incidence of monozygotic twinning was the lowest in blastocysts with grade A inner cell mass and grade B or C trophectoderm (0.82%, as the reference) and the highest in blastocysts with grade B or C inner cell mass and grade A trophectoderm (2.40%; adjusted relative risk, 2.62; 95% confidence interval, 1.60-4.43). The incidence of monozygotic twinning in blastocysts with consistent inner cell mass or trophectoderm grading was somewhere in between (both A: 1.58%; adjusted relative risk, 1.86 [95% confidence interval, 1.23-3.04]; both B or C: 1.59%; adjusted relative risk, 1.84 [95% confidence interval, 1.29-2.90]). CONCLUSION: Higher risk of monozygotic twinning was associated with blastocyst morphology specific to those blastocysts with loosely arranged inner cell mass cells combined with tightly packed trophectoderm cells.

The mystery of monozygotic twinning I: What can Amyoplasia tell us about monozygotic twinning and the possible role of twin-twin transfusion?

Amyoplasia is a very specific, nongenetic clinically recognizable form of arthrogryposis, representing about one-third of individuals with arthrogryposis surviving the newborn period. There is a markedly increased number of individuals with Amyoplasia who are one of monozygotic (MZ) twins, with the other twin being normal. Thus, it would appear that Amyoplasia is definitely associated with and may be caused by an MZ twinning event. The twin-twin transfusion seen in MZ twins could play an etiologic role in producing Amyoplasia. In this article, Amyoplasia twinning is compared to twinning in other forms of arthrogryposis. The accompanying paper examines various types of MZ twinning (Hall, 2021). Amyoplasia is primarily associated with spontaneous MZ twinning.

High grade trophectoderm is associated with monozygotic twinning in frozen-thawed single blastocyst transfer.

BACKGROUND: The aim of this study was to explore specific factors that predispose to monozygotic twinning (MZT) at the blastocyst stage. METHODS: This was a retrospective observational study of a cohort of 2863 pregnancies after single blastocyst transfer (SBT) between January 2011 and June 2019 in our hospital. MZT pregnancy was identified as the number of fetuses exceeded the number of gestational sacs (GSs) by transvaginal ultrasound at 6-7 gestational weeks. The incidences of MZT regarding the maternal age at oocyte retrieval, paternal age, ovarian stimulation protocol, fertilization method, endometrium preparation protocol, vitrified day, and the Gardner grading of the blastocyst were calculated. The serum estrogen (E2), progesterone (P) levels, endometrium thickness and serum hCG levels on day 11 after embryo transfer (ET) were compared between the MZT and singleton pregnancies. Statistical analyses were used appropriately. RESULTS: Fifty-one MZT pregnancies (1.78%) were identified. The only significant differences observed between MZT and singleton pregnancies were the proportion of TE grade (P = 0.022) and the hCG levels on day 11 after ET (P = 0.003). Multivariate logistic regression revealed that trophectoderm (TE) grade was an independent factor affecting MZT, the adjusted odds ratios (aORs) of grade A and B TE were 5.46 [95% confidential interval (CI) 1.48-20.16, P = 0.011) and 3.96 (95% CI 1.17-13.40, P = 0.027) compared to grade C respectively. There were no significant associations between the parental age, fertilization method, ovarian stimulation protocol, endometrium preparation protocol, vitrified day, expansion stage, inner cell mass (ICM) grade and MZT. CONCLUSIONS: TE grade is associated with MZT at the blastocyst stage, potentially mediated via increased secretion of hCG from more well developed TE. Increased hCG secretion in turn may prolong the implantation window to support the embryo splitting.

Monozygotic twinning following embryo biopsy at the blastocyst stage.

OBJECTIVE: Monozygotic twinning incidence following preimplantation genetic testing in embryos at cleavage-stage does not appear to increase; however, data regarding the possible impact of the blastocyst-stage preimplantation genetic testing is lacking. We compared the incidence of monozygotic twinning in preimplantation genetic testing cycles performed at the blastocyst-stage, versus cycles without PGT, following single embryo transfer. METHODS: In this retrospective cohort study, we analyzed the incidence of twin pregnancies in patients undergoing intracytoplasmic sperm injection and blastocyst-preimplantation genetic testing (253 cycles), versus a period-matched control population of patients undergoing intracytoplasmic sperm injection and single embryo transfer without preimplantation genetic testing (606 cycles). RESULTS: The overall monozygotic twinning rate was 14/859 (1.6%) per clinical pregnancy. The incidence of zygotic splitting following intracytoplasmic sperm injection and preimplantation genetic testing was 3.5% (95% Confidence interval 1.8%-6.6%) versus 0.8% (95% Confidence interval 0.3%-1.9%) following intracytoplasmic sperm injection without preimplantation sperm injection. After adjusting for potential confounders, preimplantation genetic testing cycles were associated with an increase in the incidence of monozygotic twinning when compared to cycles without embryo biopsy (Odd ratio 3.44, 95% Confidence interval 1.05-11.27, p=0.041). CONCLUSIONS: Our findings indicate that embryo biopsy for preimplantation genetic testing performed at the blastocyst stage is associated to an increase in the incidence of monozygotic twinning. Further validation in larger sample size studies is warranted. Patients undergoing preimplantation genetic testing must receive proper counselling about the potential risks of the technique.

Double trouble? Impact of frozen embryo transfer on the monozygotic twinning rate: a retrospective cohort study from 8459 cycles.

PURPOSE: To compare monozygotic twinning (MZT) rates in patients undergoing fresh embryo transfer (ET) and frozen embryo transfer. METHODS: All clinical pregnancies after single ET carried out in our IVF center between 2014 and 2018 (n = 8459) were retrospectively analyzed for the incidence of MZT. MZT rate was compared in women who underwent fresh ET (n = 3876) and frozen ET (n = 4583). RESULTS: There was a total of 120 MZT identified in the fresh ET group (3.10%) and 103 MZT in the frozen ET group (2.25%), which was significant (p = 0.015). In the univariate analysis, the risk of MZT was decreased with frozen embryo transfer (OR 0.72; 95% CI, 0.55-0.94, p = 0.016) and increased with mild stimulation protocol in the fresh cycle (OR 1.90; 95% CI, 1.04-3.45, p = 0.036). Multivariable logistic regression revealed that frozen embryo transfer was associated with a significant decrease risk of MZT (adjusted OR 0.66; 95% CI, 0.46-0.90, p = 0.011). CONCLUSIONS: Frozen ET is associated with a lower risk of MZT.

Stages of life: A new metaphysics of conceptionism.

When a human being comes into existence is crucial in bioethics. Conceptionism is the view that a human being comes into existence at conception. The twinning argument is an influential objection to this view. All versions of the twinning argument rely on a metaphysics of material objects, namely, endurantism. Given this, a strategy for defending conceptionism against the twinning argument is to deny endurantism and adopt an alternative metaphysics of material objects. A version of this strategy which has been debated in this journal is to adopt perdurantism, or the 'multiple occupancy view', on which monozygotic twins share the zygote region as a temporal part. We present a novel version of this strategy: conceptionists can evade the twinning argument by adopting an exdurantist metaphysics of material objects. We suggest reasons for thinking that this is a plausible and, indeed, preferable way for conceptionists to avoid the twinning argument.

The effect of blastocyst transfer on newborn sex ratio and monozygotic twinning rate: an updated systematic review and meta-analysis.

RESEARCH QUESTION: Is blastocyst transfer (BT) associated with a higher offspring secondary sex ratio and monozygotic twinning (MZT)? DESIGN: A systematic search of PubMed/MEDLINE, Embase, Web of Science, Cochrane Library and Google Scholar databases was carried out for studies published between 1995 and May 2017 using relevant keywords and a meta-analysis performed on selected studies. The analysis was performed using Stata 12.0; odds ratios (OR) and 95% confidence intervals (CI) were used to assess the results for binary studies. Subgroup analyses and meta-regression were also conducted. RESULTS: Twenty-six studies published between 2001 and May 2017 (sex ratio: 13, MZT: 12, and both sex ratio and MZT: 1) were identified. The analysis showed a significantly higher M/F ratio at birth (OR = 0.89, 95% CI: 0.86 to 0.93, I2 = 19.8%) and a higher risk of MZT (OR = 0.37, 95% CI: 0.22 to 0.60, I2 = 75.2%) after BT compared with cleavage-stage embryo transfer (CT). Furthermore, a subgroup analysis was performed based on studies published after 2009; results were found to be consistent with the 2009 meta-analysis. CONCLUSIONS: This meta-analysis provides an update and stronger evidence to support the observation that BT is associated with a higher proportion of males and an increased risk of MZT. In clinical practice, these BT-associated neonatal outcomes should be taken into account when counselling infertility patients.

Elevated incidence of monozygotic twinning is associated with extended embryo culture, but not with zona pellucida manipulation or freeze-thaw procedure.

OBJECTIVE: To identify the incidence and risk factors associated with IVF-conceived monozygotic twinning (MZT). DESIGN: Retrospective study. SETTING: Academic hospital. PATIENT(S): A total of 3,463 women with clinical pregnancies between January 2014 and February 2015 were analyzed. INTERVENTION(S): None. MAIN OUTCOME MEASURE(S): The measures were the incidence of MZT based on the number of embryos that were replaced, type of insemination method (conventional IVF or intracytoplasmic sperm injection [ICSI]), with or without the use of assisted hatching (AH), and day of embryo transferred in fresh and frozen cycles. RESULT(S): Ninety-three women (2.69%) with MZT were observed. No statistically significant differences were observed in the cycle parameters of fresh or frozen cycles between MZT and other non-MZT pregnancies. Specific IVF procedures or techniques, such as the number of embryo replaced, zona pellucida manipulation (ICSI and AH), and freeze-thaw procedure, did not significantly increase the rate of MZT, except for the day of embryo transferred. Compared with day 3 transferred, day 4 and 5/6 transferred showed an increased probability of MZT (odds ratio [OR], 2.73; 95% confidence interval [CI], 1.16-6.42 for day 4 transferred and OR, 3.68; 95% CI, 2.29-5.93 for day 5/6 transferred). CONCLUSION(S): Extended culture (advanced embryo stage) in fresh and frozen cycles appeared to be associated with increased rates of MZT. The effect of the number of embryos transferred, ICSI and AH, and freeze-thaw procedures on the risk for MZT was not demonstrated.

Early prenatal diagnosis of parapagus conjoined twins.

Conjoined twinning occurs in 1/100 of monozygotic twins, 1/50,000 gestations and 1/250,000 live births. It is the consequence of a division event at the primitive streak stage of the human embryonic development, about 13-14 days after fertilisation, in monochorionic monoamniotic gestations. A healthy pregnant woman, Gravida 2 Para 1, was admitted into our Fetal Medicine Unit to perform the first trimester ultrasound. A diagnosis of conjoined parapagus twinning based on ultrasound features was made at 11 weeks of gestation, and the couple decided to terminate the pregnancy. The ultrasound showed two independent skulls and hearts, a shared spine below the thoracic level, and a shared stomach. The pathological findings were slightly different, showing two independent stomachs draining into a common duodenum. The karyotype was 46 XY. Early prenatal ultrasound may provide a window to counsel the family and to offer an early termination of pregnancy.

Successful quintuplet pregnancy of monochorionic male quadruplets and single female after double embryo transfer: case report and review of the literature.

OBJECTIVE: To report a rare case of quintuplets with monochorionic male quadruplets and a single female after two-blastocyst transfer. DESIGN: Case report. SETTING: University teaching hospital. PATIENT(S): A 37-year-old gravida 1, para 1, presenting after two-embryo IVF with monochorionic male quadruplets and a single female. INTERVENTION(S): Ultrasound examinations and cesarean delivery. MAIN OUTCOME MEASURE(S): Successful delivery of five live-born infants at 28 weeks and 6 days of gestation for preterm labor. RESULT(S): The patient was diagnosed with quintuplets consisting of monochorionic male quadruplets and a single female after the placement of two embryos in blastocyst stage. She was followed closely with ultrasound examinations and hospitalized at 23 weeks' gestation. Cesarean delivery was performed at 28 weeks and 6 days of gestation, with eventual discharge of all infants in healthy condition. CONCLUSION(S): This case represents successful assisted reproductive technology quintuplets with monochorionic quadruplets and a co-sibling. Higher-order monozygotic pregnancies with monochorionic quadruplets are exceedingly rare and a potential complication of IVF.

Grade and looseness of the inner cell mass may lead to the development of monochorionic diamniotic twins.

OBJECTIVE: To examine the relationship between the inner cell mass (ICM) grade and its morphological configuration on the occurrence of monochorionic diamniotic (M-D) twinning. DESIGN: Retrospective embryo cohort study. SETTING: Private IVF clinic. PATIENT(S): Evaluation of frozen-thawed single blastocyst transfers with hormone replacement treatment in 8,435. This cohort included 71 blastocysts and their ICMs observed by time-lapse photography. INTERVENTION(S): Any changes in configuration of the ICMs observed by time-lapse photography were analyzed retrospectively. MAIN OUTCOME MEASURE(S): The amount of loosening of blastomeres within the ICM was evaluated by time-lapse observations. The number of cells that were involved in the loosening process was also assessed. Both of these parameters were correlated with the type of monozygotic twinning that eventuated. RESULT(S): The M-D twinning incidence resulting from blastocysts with a high grade ICM (grade A) were transferred was 0.38% (3/796), whereas it was significantly higher, 1.38% (34/2,463), when blastocysts with a poorer (B and C) grade ICM were transferred. Among 71 transferred frozen-thawed blastocysts that were studied with time-lapse photography, there were two dichorionic diamniotic and one M-D twins. Careful observations of the embryo that resulted in the one M-D case, revealed that the ICM acquired a looser appearance due to decompaction of at least eight cells. This type of decompaction was not observed in the ICMs of other transferred blastocysts. CONCLUSION(S): The occurrence of M-D twinning may be avoided by excluding blastocysts that contain decompacting ICMs.

The day of embryo transfer affects delivery rate, birth weights, female-to-male ratio, and monozygotic twin rate.

OBJECTIVE: To compare the reproductive outcomes between the transfer of cleavage-stage embryos and blastocysts in two different age groups of patients. The reproductive capacity of women decreases by age. This decrease in capacity is directly related to a lower ovarian reserve and errors in the meiotic spindle of the oocyte, which increase chromosomal abnormalities and the formation of aneuploidy embryos with lower chances of implantation. MATERIALS AND METHODS: A total of 1400 intracytoplasmic sperm injection cycles were analyzed. The study patients were divided into two age groups [aged < 36 years (Group I) and aged ≧ 36 years (Group II)]. The groups were subdivided according to the day of embryo transfer (ET)-Day 3 (ET3) and Day 5 (ET5). RESULTS: In both age groups, transfer of blastocysts resulted in a higher clinical pregnancy rate and deliveries. An increased twin birth rate was observed in patients who were younger than 36 years on both transfer days compared with those who were older than 36 years of age. There was an elevated percentage of newborn males on ET5 in both age groups. Monozygotic twinning (MZT) rate was observed only among younger patients (<36 years of age), specifically on ET5 compared with ET3. There was no significant difference in the mean birth weight of singleton and twins between the ET3 and ET5 subgroups in the younger group of patients except for the triplets who were significantly heavier in the ET5 group compared with the older group (≧36 years of age) where significant difference was found only on the mean birth weight of singleton. CONCLUSION: The study suggests that if a blastocyst can be obtained in patients of advanced age (≧36 years), it improves their baby take-home rates. Younger patients (aged < 36 years) should undergo elective single blastocyst transfers to reduce multiple pregnancy rates.

Monozygotic twinning after assisted reproductive technologies: a case report of asymmetric development and incidence during 19 years in an international group of in vitro fertilization clinics.

OBJECTIVE: To describe a case of monozygotic twinning with asymmetric development following a single fresh embryo transfer as part of an intracytoplasmic sperm injection (ICSI) treatment. Secondarily, to report the incidence of monozygotic twinning at the IVI (Instituto Valenciano de Infertilidad) clinics. DESIGN: Case report. SETTING: Private fertility centers. PATIENT(S): A 33-year-old woman with a 2-year history of primary infertility. INTERVENTION(S): Controlled ovarian hyperstimulation and ICSI treatment with single-embryo transfer. MAIN OUTCOME MEASURE(S): Incidence of monozygotic twinning at the IVI clinics. RESULT(S): We report a twin pregnancy after a single-embryo transfer. Twins were dichorionic and diamniotic. One fetus had a 6-day delay in its growth compared with the other when observed by ultrasound. Two female infants were delivered, and despite presenting congenital diseases, they were successfully treated and evolved correctly. A subsequent DNA analysis confirmed that the infants were monozygotic. Furthermore, we estimated a monozygotic twinning rate of 1.17% at the IVI clinics, taking into account those cases in which two or more embryos with heart beats were observed by ultrasound scanning after single-embryo transfers. CONCLUSION(S): Ultrasound scans performed during pregnancy suggested a possible dizygotic origin of the twins, but DNA analysis performed after birth established that they were monozygotic. Genetic analysis is the only valid tool to confirm if like-sex dichorionic twins are monozygotic or dizygotic.

Monozygotic twinning after in vitro fertilization/intracytoplasmic sperm injection treatment is not related to advanced maternal age, intracytoplasmic sperm injection, assisted hatching, or blastocyst transfer.

OBJECTIVE: To evaluate the effect of assisted reproductive techniques on the incidence of monozygotic twins (MZT) and the associated pregnancy outcomes. MATERIALS AND METHODS: This was a retrospective study of all in vitro fertilization (IVF)/intracytoplasmic sperm injection (ICSI) cycles with MZT pregnancies in our center from January 2001 to December 2011. The diagnosis of MZT pregnancies with their respective placental configurations was based on the results of ultrasonographic examinations performed during either the first or second trimester. The treatment characteristics and outcomes of each IVF cycle were recorded and stored in a computer database. RESULTS: A total of 17 cycles with MZT pregnancies were identified, resulting in an overall incidence of MZT of 1.3%. The incidence of MZT for women aged <35 years and ≥35 years were 1.5% and 0.8%, respectively (p = 0.319). The incidence was not significantly different between ICSI and non-ICSI cycles (1.4% vs. 1.0%; p = 0.620). In addition, the incidence was not increased in the assisted hatching (AH) group compared to those without AH (0.9% vs. 2.1%; p = 0.103). Finally, cycles with embryo transfer at the blastocyst stage had an MZT incidence that was not significantly different from those transferred at the cleavage stage (1.4% vs. 1.3%, respectively; p = 1.000). The incidence of each type of chorionicity, dichorionic-diamniotic, monochorionic-diamniotic, and monochorionic-monoamniotic, was 33.3%, 46.7%, and 20.0%, respectively. A total of 11 of 39 (28%) monozygotic babies and 16 of 19 (84%) coexisting heterozygotic babies were born alive. CONCLUSION: Until definite conclusions are drawn from larger trials, patients receiving IVF should not be overly concerned about the increase in MZT risk when proceeding to various assisted reproductive procedures (i.e., ICSI, AH, and blastocyst transfer). However, there is some evidence that the incidence of monochorionic-monoamniotic twins may be significantly increased after IVF/ICSI cycles. Patients should be informed about the possible obstetric complications regarding this rare type of MZT.