Testosterone in the maternal bloodstream follows a similar pattern, increasing slowly throughout pregnancy with no fetal gender-related differences. Actually, maternal blood oestrogen levels rise steadily throughout pregnancy whatever the sex of the fetus, although female fetuses do show a higher oestrogen concentration in the amniotic fluid early in the second trimester.
There’s a lay belief that when pregnant with a girl, maternal oestrogen levels are higher, but this is not backed up by most studies. There are plenty of other hormones at work in pregnancy, whatever the sex of the fetus. Researchers have found that severe morning sickness (called hyperemesis gravidarum) is slightly more common in pregnancies where the baby is a girl, which could reflect the differences in hCG. The surge of hCG early in pregnancy is one explanation for the less desirable symptoms of pregnancy, such as nausea and sickness. Some studies only report this later in pregnancy. Studies have shown hCG (human chorionic gonadotropin, the hormone responsible for the second line appearing on a home pregnancy test) is higher for female fetuses compared with males, and remains higher throughout pregnancy.
There is evidence hormone concentrations in pregnancy can differ according to the sex of the fetus as early as three weeks after fertilisation. Hormone differences for baby boys and girls