Ultrasound examination of the fetal perinea after 30 weeks of gestation can accurately determine fetal sex with a 95.6% accuracy rate.

Fetal Sex Determination by Ultrasound

Sex differences in the placenta contribute to altered fetal growth and developmental outcomes, with males prioritizing growth pathways but less adaptable to changes in the in-utero environment, increasing their risk for intrauterine morbidities or mortality.

Let’s Talk about Placental Sex, Baby: Understanding Mechanisms That Drive Female- and Male-Specific Fetal Growth and Developmental Outcomes

Sexual dimorphic differences in pre-eclampsia occurrence exist, with preterm PE being more prevalent in pregnancies with a female fetus compared to male fetuses, but no differences in term PE occurrence.

Fetal sex-specific differences in gestational age at delivery in pre-eclampsia: a meta-analysis

Fetal sex influences pregnancy and delivery outcomes, with male fetuses at higher risk of preterm labor and female fetuses at higher risk of hypertension and type 2 diabetes post-pregnancy.

Effect of Fetal Sex on Maternal and Obstetric Outcomes

Male and female fetuses respond differently to the same intrauterine environment, suggesting a fundamental biological variation at the cellular and molecular level.

Sex Differences in Nutrition, Growth, and Metabolism in Preterm Infants

Carrying a female fetus decreases the risk of insulin resistance in the mother, as early as the first trimester.

The Effects of Fetal Gender on Maternal and Fetal Insulin Resistance

Fetal mouth movements increase with gestational age, with females showing more mouth movements than males, suggesting a more advanced female development pattern than males.

Sex differences in fetal mouth movements

Women carrying a boy have a 4% higher risk of gestational diabetes mellitus compared to those carrying a girl, suggesting the fetus may have unsuspected effects on maternal glucose metabolism in pregnancy.

Sex of the baby and risk of gestational diabetes mellitus in the mother: a systematic review and meta-analysis

Females exhibit unique vulnerabilities and developmental consequences after prenatal immune disruption, highlighting the need to avoid extrapolating male disease mechanisms to females.

“Females Are Not Just ‘Protected’ Males”: Sex-Specific Vulnerabilities in Placenta and Brain after Prenatal Immune Disruption

Maternal asthma during pregnancy causes a sexually dimorphic effect on fetal growth, with female fetuses altering placental glucocorticoid metabolism and decreasing growth, while male fetuses continue to grow unaffected by maternal asthma.

Sexually dimorphic effects of maternal asthma during pregnancy on placental glucocorticoid metabolism and fetal growth

Male sex is an independent risk factor for adverse pregnancy outcomes, with females having a better perinatal outcome than males, particularly after preterm birth.

Does fetal sex affect pregnancy outcome?

Human females outlive males at every biological stage, but more boys are born than girls, due to significant loss of females before the fetus arrives in the womb and excessive loss of males throughout their lives.

The Human Female Paradox: Biological Disadvantage Preimplantation, Biological Advantage, Thereafter

Day 18 and possibly day 19 post conception are critical for priming the male central nervous system with high testosterone levels, followed by exposure to relatively low testosterone levels, completing masculinization.

Plasma testosterone and progesterone titers of pregnant rats, their male and female fetuses, and neonatal offspring.

Fetal gender is unlikely to significantly affect maternal asthma during pregnancy, requiring more acute care and medications for women pregnant with a female fetus.

Effect of fetal gender on maternal asthma exacerbations in pregnant asthmatic women.

Intrauterine position during fetal life influences postnatal reproductive traits, with testosterone concentrations and estradiol concentrations playing crucial roles in sexual differentiation in rodents and swine.

Sexual differentiation in litter-bearing mammals: influence of sex of adjacent fetuses in utero.

Male gender significantly predisposes to prolonged pregnancy, with by 43 weeks of gestation, there are 3 male deliveries for every 2 female deliveries.

Male gender predisposes to prolongation of pregnancy.

Having a male fetus is associated with poorer -cell function, higher postprandial glycemia, and an increased risk of gestational diabetes mellitus in the mother.

Fetal Sex and Maternal Risk of Gestational Diabetes Mellitus: The Impact of Having a Boy

High sex ratios at birth can negatively impact fertility patterns and community development, with primiparous women having more female babies than multipara women.

Sex Ratio at Birth: Where Did Our Female Fetus Go?

Pregnancies with a male fetus may generate a more pro-inflammatory environment in the placenta, potentially impacting preterm birth rates and fetal/newborn wellbeing.

Fetal sex and preterm birth.

The birth of a boy may immunize some women against male (Y) antigen, potentially harming subsequent male fetuses and potentially inactivating Y-bearing sperm before fertilization.

Does Maternal Immunity to Male Antigen affect the Sex Ratio of the Young?

These studies suggest that not every fetus is female, as fetal sex can be determined with high accuracy and there are sex-specific differences in fetal development.