Effects of foetal sex on maternal physiology

I queried (lamented really) in a previous post whether your unborn baby’s sex affects your strength/ fitness (I was much stronger in my previous pregnancy (boy) than my current one (girl) and wasn’t sure if this was due to the foetal sex, or other factors – sleep deprivation, less time devoted to training, etc).

Turns out there’s a real dearth of literature relating to¬†this topic…

Overall, the ‘apparent’ effects of foetal sex on maternal status during pregnancy can be summarised:*

If you are PREGNANT WITH A MALE BABY, you are more likely to:

  • Be anxious or angry during pregnancy – definitely not true for my first pregnancy, I was super zen ūüôā
  • Have better spatial skills during pregnancy
  • Have a tougher, more dominant personality
  • Tolerate risky behaviours in your children
  • Take risks yourself
  • Suffer from post-natal depression
  • Have better asthma control during pregnancy (if you already have asthma)

If you are PREGNANT WITH A FEMALE BABY, you are more likely to:

  • Remain calm during pregnancy (I have never had more meltdowns than early in this pregnancy, therefore I would categorically reject this claim! Level V evidence, if that!)
  • Be more empathetic during pregnancy
  • Have worsened asthma during pregnancy (if you already have asthma)

* PLEASE be aware that I cannot vouch for the source of this information or the quality of the studies backing it… The studies that I could find were mostly poor quality and low-level…

If you are willing to take ‘facts’ at face value, you can stop reading here. However, if like me you were like “WTF”, then please read on…

Higher risk of post-natal depression (PND) in mothers of boys: a study in Evolutionary Psychology (2008) found that mothers of boys not only have more PND, but they are likely to report a lower quality of life irrespective of whether they are classified as depressed or not. No physiological basis is given for this (it is a more observational study) other than “male babies are perceived as being more difficult”. If you’re like me, with an ‘easy’ male baby, you would disagree!

Improvement in asthma in women pregnant with male babies: if you are pregnant with a male baby, you are likely to experience ~10% improvement in airway stability during pregnancy (compared with carrying a female baby). This is thought to be due to the surge of androgens at 12-16 weeks, giving a protective effect on the airways. Additionally, sex-specific factors related to the a female foetus may promote activation of inflammatory pathways, and hence a worsening of maternal asthma.

Effect of male foetal sex on cardiovascular risk factors and diabetes control: a study looking at over 2000 pregnant women found that those delivering a boy had lower systolic and diastolic blood pressure, and lower HbA1c (a marker for glycaemic control) compared with those delivering girls (after controlling for other variables). It is said to do this via foetal sex hormone differences affecting maternal hormone receptor receptivity. OK, if you say so. Interestingly however, it has been found that mothers with male babies are at a slightly increased risk of gestational diabetes (GDM) than mothers of girls (7% vs. 3% increase); this is due to the effect of the male foetus on the mother’s ability to produce insulin, which is the main determinant of whether a pregnant woman will develop GDM or not.

Improved milk production with female babies (well, in cows anyway…): mothers of girl babies produce more milk than boy babies (although overall quality is the same). The authors explain this in terms of evolutionary biology – with more milk for daughters meaning they mature faster and are therefore able to breed earlier (like I said, for cows), thereby ensuring continuation of the species. In that respect, mothers are able to ‘under-invest’ in their sons because even with undernourishment, they are still able to pass on their genes.

Differing hormone levels¬†associated¬†with girl and boy babies: placentas differ according to the baby’s sex as well… This is because the placental is primarily derived from embryonic dissue and has the same genetics as the foetus. Female foetal placentas have a greater exchange region compared with males, and male foetal placentas have higher levels of glucose transporter (hence the relationship with increased chance of GRM) and EGF binding protein. There are many other differences between the placentas of female and male babies, which I won’t go into because they are hard-core specialised and way beyond the purpose of our discussions here.

Higher maternal testosterone levels with male foetuses: testosterone is produced by male babies from week 6 onwards, and babies up to 5 months of age can have the same amount of testosterone as a pubertal male. Cue porno mo and crotch grabbing. Testosterone diffuses through the amniotic fluid and also transfers via the mother’s blood stream. This increased exposure to testosterone is thought to account for the mother’s improved spatial ability and more irritable/ aggressive mood when pregnant with male babies.

What is interesting for the last point is that there is a lot of evidence that women with higher testosterone levels are more likely to birth a boy than a girl. It is thought that her own natural levels of testosterone make the reproductive environment such that it favours development of a boy, rather than a girl. This evidence stems from evidence that boisterous, energetic, powerful women are in general more likely to have boy babies. Additionally, women with the same characteristics, if they have girl babies, are more likely to have girls with more tomboy-like features.

Anyway, the purpose of this post was to see if foetal sex had any effect on maternal strength / fitness. I couldn’t find ANYTHING that specifically addressed this question. The best I can do is extrapolate: ¬†if testosterone is higher in women pregnant with boys, then technically, this should enhance strength and training ability.

If you have any facts/ figures/ research addressing the effect of foetal sex on maternal strength and fitness, please share!

Helen L. Kwon , Kathleen Belanger , Theodore R. Holford , and Michael B. Bracken. Effect of Fetal Sex on Airway Lability in Pregnant Women with Asthma. American Journal of Epidemiology, Vol. 163, No. 3 .
Hocher B, Chen YP, Schlemm L, Burdack A, Li J, Halle H, Pfab T, Kalk P, Lang F, Godes M. Fetal sex determines the impact of maternal PROGINS progesterone receptor polymorphism on maternal physiology during pregnancy. Pharmacogenet Genomics. 2009 Sep;19(9):710-8. doi: 10.1097/FPC.0b013e328330bc7a.
Hinde, K., Carpenter, A., Clay, J., Bradford B. 2013. Holsteins Favor Heifers, Not Bulls: Biased Milk Production Programmed during Pregnancy as a Function of Fetal Sex. PLOS One.
Ravi Retnakaran, Caroline K. Kramer, Chang Ye, Simone Kew, Anthony J. Hanley, Philip W. Connelly,Mathew Sermer, and Bernard Zinman. Diabetes Care, February 18, 2015

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